The development of wireless metropolitan area networks (WMAN's) and wireless local area networks (WLAN's) for broadband wireless access (BWA) to voice and data telecommunication services is an area of considerable economic and technological interest. The WMAN systems typically employ a point-to-multipoint topology for a cost effective system deployment. For example, proposed WMAN systems operating in the 2 to 6 GHz radio frequency (RF) range consist of base station cell tower sites with 3 to 6 antenna/transceiver sectors, with capacity goals of 40 to 80 Megabits per sector, and with coverage goals of 5 to 15 kilometer cell radius. Example WLAN systems include installations at areas both inside and outside of residences or businesses and public areas such as trains, train stations, airports or stadiums. These WMAN and WLAN systems can also be integrated to form a wide area network (WAN) that can be national or even global in coverage. WMAN systems are primarily discussed because they are technically the most challenging. However, the invention may also be used in broadband wireless access systems in general.
The primary problem in broadband wireless telecommunication is the considerable variation in the quality of the RF reception. The RF reception varies due to the type of terrain, due to the presence of obstacles between the base station and the subscriber station (SS), and due to the fairly high probability of receiving the same transmission by means of multiple RF propagation paths. The latter problem is referred to as “multipath” and the above set of reception problems is often collectively, and loosely, referred to as the non-line-of-sight (NLOS) reception problem. When the SS is moving, there is the additional problem of Doppler induced channel variability. A robust NLOS BWA system for fixed or mobile subscribers is a technical challenge.
The WMAN systems of interest typically have RF channels that are the composite of multiple radio propagation paths over large distances. A consequence of these multipath propagation channels is that the received radio signal waveforms are distorted relative to the original transmitted radio signal waveforms. Prior art high data rate WMAN signaling technologies that are intended to mitigate the multipath performance degradations are orthogonal frequency division multiplexing (OFDM) and single carrier with frequency domain equalization (SC-FDE).
FIGS. 1 and 2 are diagrams of known OFDM and SC-FDE methods of transmitting and receiving signals with digital data modulations through dispersive propagation channels that impose some degree of multipath signal distortions. These diagrams emphasize the method specific signal processing elements and illustrate their dependence on FFT block processing.
FIG. 1 shows a block diagram illustrating certain processes performed by a system implementing the OFDM method. An inverse fast Fourier transform (inverse FFT) 110 transforms the data (symbols) 108 to be transmitted. Cyclic prefix insertion process 112 creates a serial output block with ends that are circular in content. These processes occur within OFDM transmitter 114. The transmitter output 115 passes through a propagation channel 116 to become input 117 to OFDM receiver 118. The method specific processes the OFDM receiver include a forward fast Fourier transform (FFT) process 120 that creates intermediate data symbols that have been distorted by the propagation channel, a process 122 to invert the channel and a process 124 to detect the original symbols, i.e., to provide the received data output 126. The OFDM symbol detection process 124 may, for example, include Viterbi decoding, symbol de-interleaving, and Reed-Solomon forward error detection/correction (FEC). The specific detection process 124 depends on the coding/interleaving that was applied to the transmitted data symbols 108.
FIG. 2 shows a block diagram illustrating certain processes performed by a system implementing an SC-FDE method. The data symbols to be transmitted 128 are input to a preamble and cyclic prefix insertion process 130. The preamble sequence has good correlation properties to support channel estimation and the cyclic prefix insertion creates circular output blocks to simplify receiver FFT operations. These processes occur within SC-FDE transmitter 132. The transmitter output 133 passes through a propagation channel 134 to become input 135 to SC-FDE receiver 136. The method specific processes in SC-FDE receiver 136 include a forward FFT 138 process to transform the signal into the frequency domain, a frequency domain filter to invert the channel 140, an inverse FFT 142 to restore the signal to the time domain, and symbol detection 144 that provides the received data output 146. The SC-FDE symbol detection process 144 may include a non-linear decision feedback equalizer (DFE) in addition to decoding and de-interleaving operations. As in the OFDM method, the detection process 144 may, for example, include Viterbi decoding, de-interleaving, and a Reed-Solomon FEC, or functionally similar operations, depending on the coding/interleaving that was applied to the transmitted data symbols 128.
Operationally, the OFDM and SC-FDE systems differ mainly in the placement of the inverse FFT. In the OFDM method the inverse FFT is at the transmitter to code the data into the sub-carriers. In the SC-FDE method the inverse FFT is at the receiver to get the equalized signal back into the time domain for symbol detection. Although FIG. 2 shows the SC-FDE signal to have a cyclic prefix insertion 130, this is actually an option for SC-FDE that trades useable bandwidth for a slightly decreased number of receiver computations and a potential performance improvement. In the OFDM method, the cyclic prefix insertion 112 and the associated loss of useable bandwidth are mandatory.
For high data rate single carrier (SC) systems, WMAN multipath RF channel distorts the signal by mixing data symbols that were originally separated in time by anywhere from a few symbols to a few hundreds of symbols. This symbol mixing is referred to as inter-symbol interference (ISI) and makes the SC wireless link useless unless equalization is performed. It is generally agreed that traditional time domain adaptive equalization techniques are impractical to solve this problem since the computations per bit are proportional to the ISI span, which in the WMAN channels of interest can be hundreds of symbols. However, the FFT can be used to provide efficient frequency domain equalization for single carrier signaling. This is the basis of the single carrier frequency domain equalization (SC-FDE) method discussed above. SC-FDE is known to work well in terms of multipath mitigation and is practical in terms of transceiver computations per bit. A modem SC-FDE method is described by David Falconer, Lek Ariyavisitakul, Anader Benyamin-Seeyar and Brian Eidson in “Frequency Domain Equalization for Single-Carrier Broadband Wireless Systems”, IEEE Communications Magazine, Vol. 40, No. 4, April 2002.
For high data rate OFDM systems, WMAN multipath RF channels often result in severe spectral nulls. These spectral nulls make the OFDM wireless link useless unless interleaving and coding are performed. Coherent OFDM also requires equalization. However, OFDM with interleaving, coding, and equalization is known to work well in terms of maintaining a WMAN link in the presence of multipath and is equivalent to SC-FDE in terms of transceiver computations per bit. A critical comparison of the OFDM and SC-FDE techniques is given by Hikmet Sari, Georges Karam and Isabelle Jeanclaude in “Transmission Techniques for Digital Terrestrial TV Broadcasting”, IEEE Communications Magazine, February 1995.